Pressure-inductance transducer

ABSTRACT

In an electronic fuel injection system, a transducer for converting a variable pressure signal to a variable inductive signal has a coil wound on a plastic spool and surrounded by a ferrite cup and cap. A nickel plated steel sleeve snaps into the spool and supports a Teflon coated steel plunger which is tapered to provide a variable air gap with the ferrite cap. This assembly is resiliently supported in a housing and is operated through a bimetallic disc from an evacuated bellows responsive to the absolute pressure in the engine air induction passage. The bellows is positioned by a diaphragm which also is responsive to induction passage pressure to provide power enrichment. Adjustments are provided for the idle, power cut-in, and wide open throttle points and for initial calibration of the transducer.

United States Patent [1 1 Moulds et al.

[ PRESSURE-INDUCTANCE TRANSDUCER 2,437,345 3/1948 Bell 336/179 X 7 7 6 7 [75] Inventors: John W. Moulds, Penfield; Edwin C. 4835 5/1956 Gama 33 ll 9 X Storey, Rochester, both of NY. FOREIGN PATENTS OR APPLICATIONS [73] Assignee: General Motors Corporation, 1 213, 3 gig: 336/134 Detroit, Mich. [22] Filed: Jan. 22, 1973 Primary Examiner-Thomas .l. Kozma [21] Appl. No.: 325,861

[57] ABSTRACT Related Application Data In an electronic fuel injection system, a transducer for [62] of 1971 converting a variable pressure signalto a variable inductive signal has a coil wound on a plastic spool and surrounded by a ferrite cup and cap. A nickel plated [52] U.S. Cl 336/30,:333366/l8336, 333366/l13 ;i9, Steel sleeve Snaps into the Spool and supports a Teflon 51 I t Cl H0 4 b 06 coated steel plunger which is tapered to provide a 30 179 83 variable air gap with the ferrite cap. This assembly is le 0 l resiliently supported in a housing and is operated through a bimetallic disc from an evacuated bellows responsive to the absolute pressure in the engine air I [561' References cued induction passage. The bellows is positioned by a dia- UNITED STATES PATENTS phragm which also is responsive to induction passage 2,735,368 2/1956 Antonazzi 336/30 X pressure to provide power enrichment. Adjustments 3,221,281 11/1965 are provided for the idle, power cut-in, and wide open 2,664,795 1/1954 throttle points and for initial calibration of the trans- 3,308,41 I 3/1967 ducer. 2,707,001 4/1955 Hathaway 336/179 X 3,308,412 3/1967 Curtis et al. 336/179 X 7 Claims, 2 Drawing Figures x .754 jg 4 /o 7;: X 22 #05 41 /Z a /3' w 1 33 I n n n a l JPRESSIURE 1 i V "/6 fli :29 6.? 5%

[4 1 Mar. 26, 1974 posals. This invention provides a pressure-inductance transducer of improved construction which 'will provide a more accurately controlled and more reliable inductive signal to an electronic fuel injection system.

Among the features of this transducer: The inductive coil is disposed in a cylindrical ferrite cup having a cir cular base and cap which provides a symmetrical flux path with low eddy current losses. A steel plunger has a tapered end associated with an opening in the ferrite cap to provide a variable air gap which controls the reluctance of the flux path and thus the inductance of the coil. The plunger slides in a steel sleeve which is snapped into a plastic spool about which the inductive coil is wound, the sleeve extending through an opening in the base of the ferrite cup; the sleeve provides an im proved flux path between the ferrite cup and the plunger and at the same time provides a precisely aligned, stable bearing for the plunger. The sleeve is nickel plated and the plunge is Teflon coated to minimize friction therebetween.

As additional features: The aforedescribed inductive unit is resiliently mounted in a hollow housing by a ring having a plurality of radially extending fingers which engage the housing walls. Silicone rubber wafers are disposed between the ring and the ferrite cap and between the base of the ferrite cup and the housing. This mounting arrangement prevents axial and radial displacement of the inductive unit and prevents axial compression of the unit.

As has been proposed heretofore, the plunger is positioned by an evacuated bellows responsive to the absolute pressure in the engine air induction passage below the throttle. Another novel feature of this transducer is a bimetallic disc which connects the bellows to the plunger; the disc compensates for changes in plunger position, coil resistivity and flux path permeability due to changes in temperature.

The evacuated bellows, being responsive to the absolute pressure in the induction passage below the throttle, positions the plunger in accordance with air flow to the engine so that an inductive signal proportional to engine air flow is created. Under conditions of high air flow, however, it is desirable to increase the inductance a step above that provided by the evacuated bellows alone. To accomplish this, a diaphragm allows shifting of the bellows and plunger a selected amount when the manifold pressure rises above a selected level. This permits power enrichment at high rates of air flow.

A calibration screw is provided to permit initial setting of the position of the plunger, and adjusting screws also are provided to limit travel of the plunger at low andhigh manifold pressures and to establish the pressure level at which the diaphragm permits the bellows and plunger to shift.

The details as well as other objects and advantages of this invention are described below and are set forth in the drawing in which:

FIG. 1 is an axial sectional view through the pressureinductance transducer provided by this invention; and

FIG. 2 is a schematic diagram showing how the transducer may be incorporated in an electronic fuel injec tion system.

Referring first to FIG. 1, a transducer 10 includes a die cast housing 12 defining a chamber 14 having a signal passage 16 through which a pressure signal may be applied. An inductive unit 18 is disposed within chamber 14 and includes a ferrite cup 20 having a cylindrical peripheral wall 22 and a circular base or end wall 24.- A circular ferrite cap 26 is bonded to cup 20.

A nylon spool 28 is disposed within cup 20 and has an inductive coil 30 wound thereon. Coil 30 has leads 32 and 34, the latter of which may be grounded by a screw 36 to housing 12 and thence to the engine block if so desired.

Spool 28 has a central bore 38 extending therethrough, bore 38 having a larger diameter on the righthand end than on the left-hand end, as shown, to define a shoulder 40 therebetween. A projection 42 near the right-hand end of the bore 38, extends radially into bore 38.

A steel sleeve 44 is inserted in bore 38 until its lefthand end abuts shoulder 40 and projection 42 is received in an annular recess 46 formed about sleeve 44. The right-hand end of sleeve 44 extends out through an opening 48 in the base 24 of cup 20 and is received in a recess 50 in the end wall 52 of housing 12. The insertion of sleeve 44 in recess 50 helps'to align inductive unit 18 within housing 12.

Inductive unit 18 is retained in housing 12 by a ring 54 which has a plurality of spring fingers 56 engaging the peripheral wall 58 of housing 12. Ring 54 is backed by a silicone rubber wafer 60, and a silicone rubber wafer 62 is disposed between end wall 24 of cup 20 and a raised portion 64 of housing end wall 52. Ring 54 and wafers 60 and 62 serve to prevent axial or radial displacement of inductive unit 18 within housing 12, and wafers 60 and 62 further prevent axial compression of inductive unit 18 as ring 54 is pressed into housing 12.

A steel plunger 66 is received and guided in the bore 68 of sleeve 44, sleeve 44 being nickel plated and plunger 66 being Teflon coated to minimize frictional hysteresis. Steel plunger 66, steel sleeve 44, ferrite cup 20 and ferrite cap 26 provide a flux path through and about coil 30. The symmetry of cup 20 and cap 26 about plunger 66 provides an improved flux path, and the use of ferrite for the cup 20 and cap 26 provides a low eddy current loss flux path as well as providing an easily processable material for economy of production. Provision of sleeve 44 improves flux transfer between the base 24 of cup 20 and plunger 66 and in addition provides a guide and bearing for plunger 66 which is stable over the operating temperature range. When steel sleeve 44 is used to support and guide plunger'66, the manufacturing tolerances of spool 28 may be eased considerably.

A brass extension 70 is threadedly received, as at 72, by plunger 66 and has an enlarged portion 74 the left side of which defines a surface 76 facing away from plunger 66. A pin 78 extends leftwardly from surface 76. A bimetallic disc 80 has a central aperture 82 received on pin 78 and an outer periphery 84 received by the rim 86 of a plate 88. The high expansion side of biinclude an internal spring 92 to bias bellows 90 in an expansion mode.

In operation, bellows 90 is subjected to the induction passage pressure communicated to chamber 14 and acts through plate 88, disc 80 and extension 70 to move plunger 66. As the induction passage pressure increases with increasing air flow, bellows 90 is compressed and plunger 66 is pushed leftwardly by a spring 94. This motion moves the tapered end 96 of plunger 66 toward ferrite cap 26, reducing the air gap between plunger 96 and the edge of an opening 98 in cap 26 and thus increasing the inductance of coil 30. As the induction passage pressure falls with decreasing air flow, bellows 90 expands with the assistance of spring 92 and plunger 66 is pushed rightwardly against the bias of spring 94.

This'motion increases the air gap between the tapered end 96 of plunger 66'and the edge of opening 98 and thus reduces the inductance of coil 30.

Upon an increase in temperature, bimetallic disc 80 flexes convexly toward the right, thereby shifting plunger 66 rightwardly to compensate for a reduction in the air gap between plunger 66 and the edge of opening 98 due to thermal expansion in the transducer and 7 also to compensate for a decrease in coil resistance and an increase in flux path permeability due to the temperature increase. In the illustrated embodiment, bimetallic disc 80 may be designed to providea travel of 0.02 inch against a 2 lb. force upon a temperature increase from 75 to 250F.

The left side of bellows 90 abuts a calibration screw 100 which is adjustable, against the resiliency of bellows 90 with its spring 92 and the bias of spring 94, to permit initial positioning of plunger 66.

Calibration screw 100 is carried on a diaphragm 102 which is biased leftwardly by a helically coiled spring 104. The other end of spring 104 is seated on a ring 106. On the left side, diaphragm 102 is subjected to atmospheric pressure which is applied through an opening 103 in a cover plate 110. On the right side, diaphragm 102 is subjected to the induction passage pressure in chamber 14. When the induction passage pressure drops below a predetermined level, diaphragm 102 is displaced toward the right against the bias of spring 104, thereby shifting bellows 90 and plunger 66 toward the right against the bias of spring 94. This decreases the inductance of coil 30 a selected value below that achieved-by expansion of bellows 90 which also moves plunger 66 toward the right, thereby reducing the fuel flow provided by the associated fuel injection system for economical operation. Motion of diaphragm 102 toward the right is limited as an annular stop 112 is contacted by a diaphragm backing plate 114. When the induction passage pressure rises above the predetermined level an indication of the need for an enriched air-fuel mixture, diaphragm 102 is moved leftwardly by spring 104, thereby allowing leftward movement of bellows 90 and plunger 66 under the bias of spring 94. This increases the inductance of coil 30 a selected value over and above that achieved by compression of bellows 90 which also moves plunger 66 toward the left, thereby increasing the fuel flow provided by the associated fuel injection system to provide an enriched mixture for power operation.

The value of induction passage pressure at which diaphragm 102 is shifted toward the right may be adjusted by threadedly turning ring 106 within the member 116, screw slots 118 being provided in ring 106 for this purpose. After factory adjustment, ring 106 is staked in place.

An adjusting screw 120, received in a bushing 122 secured to plate 110, limits leftward motion of diaphragm 102. Stop 120 is threadedly adjustable with respect to bushing 122 to establish the amount of power enrichment which may be caused by diaphragm 102,

Referring now to FIG. 2 where the fuel injection system is schematically illustrated, the induction passage has a throttle 142 which controls air flow to the engine. It will be noted that the inlet passage 16 to transducer 10 connects with induction passage 140 downstream of throttle 142 to sense the induction passage pressure therebelow. An injector nozzle 144 is disposed to discharge into induction passage 140 adjacent the I inlet valve 146 for the engine combustion chamber. A separate nozzle 144 would ordinarily be provided for each combustion chamber.

The electronic control circuit includes a unit 148 which applies a negative pulse to line 150 each time solenoid operated nozzle 144 is to be energized that is, each time fuel must be supplied for combustion in the associated combustion chamber. The negative pulse renders an output transistor 154 nonconductive. With output transistor 154 off, current flows through a solenoid winding 156 to energize throttle 144 and initiate fuel injection. Nozzle 144 remains energized for a period of time determined by the remainder of the electronic control circuit.

The negative pulse also renders a control transistor 157 nonconductive. With control transistor 157 off, the voltage at a junction 158 increases and renders a transistor 160 conductive which in turn renders a transistor 162 nonconductive. With transistor 162 off, transistors 164 and 166 also are rendered nonconductive. As long as transistor 166 is off, output transistor 154 remains off and permits current flow through solenoid 156 to energize injection nozzle 144.

The aforementioned transistors are maintained in the indicated state for a period of time determined by the L/R time constant provided by the inductance of transducer coil 30 and the combined resistances of the resistors 168.and 170. When the time constant permits decay of the voltage at junction 158 to a predetermined level, transistor 160 is rendered non-conductive and transistor 162 is thereby rendered conductive. With transistor 162 on, transistors 164 and 166 are rendered conductive. As transistor 166 becomes conductive, output transistor 154 becomes conductive, preventing current flow through solenoid 156 to de-energize nozzle 144 and terminate fuel injection.

Thus the duration of fuel injection is controlled by the level of a voltage signal at junction 158 which in turn is controlled by an L/R time constant. In the sim plified electronic control circuit shown here for purposes of illustration, this time constant is varied solely by varying the inductance of transducer coil 30. As explained above, this inductance varies with the pressure in induction passage 140 below throttle 142. As the pressure increases with increasing air flow, the inductance increases to increase the duration of fuel injection and thus to increase fuel flow, and as the pressure decreases with decreasing air flow, the inductance decreases to decrease the duration of fuel injection and thus to decrease fuel flow.

We claim: 7

l. A transducerfor converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening adjacent the large diameter end thereof,

a coil wound about said spool,

a steel sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annular recess receiving said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool, ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said other end of said sleeve,

a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening,

steel plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap,

said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap,

and pressure responsive means connected to said plunger for axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil.

2. A transducer for converting a variable pressure signal to a variable inductive signal comprising:

an inductive coil,

a plunger disposed within said coil,

means associated with said plunger for defining a flux path through and about said coil,

pressure responsive means for axially moving said plunger within said coil to thereby vary the inductance of said coil,

and bimetallic temperature responsive means connecting said pressure responsive means to said plunger for varying the axial position of said plunger with respect to said pressure responsive means to thereby compensate for variations in plunger position, coil resistance, and flux path permeability due to temperature variations.

3. A transducer for converting a variable pressure signal to a variable inductive signal comprising:

an inductive coil,

a plunger disposed within said coil,

means associated with said plunger for defining a flux path through and about said coil, said plunger being axially movable within said coil to thereby vary the inductance of said coil,

a member connected to said plunger and having a radially extending surface facing away from said 4. A transducer for converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough and an inductive coil wound thereabout,

a sleeve received in said opening, one end of said sleeve extending axially beyond the end of said spool,

a ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said end of said sleeve,

a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, v

a plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap,

said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap,

a cupped housing having an end wall and a peripheral wall, said end wali of said ferrite cup being disposed adjacent said end wall of said housing and said peripheral wall of said housing surrounding said cylindrical wall of said ferrite cup, said end wall having an axial recess receiving and aligning said sleeve,

a retainer ring disposed adjacent said ferrite cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall,

a resilient wafer disposed between said end wall of said ferrite cup and said end wall of said housing, and a resilient wafer secured to said retainer ring between said ring and said cap, whereby said ferrite cap and cup are retained against radial displacement and cushioned against axial compression in said housing,

and pressure responsive means connected to said plunger for axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil.

5. A transducer for converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening ad jacent the large diameter end thereof,

an inductive coil would about said spool,

a sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annu lar recess receiving'said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool,

a cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said other end of said sleeve,

a circular cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, I

a plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap, said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap,

a cupped housing having an end wall and a peripheral wall, said end wall of said cup being disposed adjacent said end wall of said housing and said peripheral wall of said housing surrounding said cylindrical wall of said cup, said end wall having an axial recess receiving and aligning said sleeve,

a retainer ring disposed adjacent said cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall,

a resilient wafer disposed between said end wall of said :cup and said end wall of said housing and a resilient wafer secured to said retainer ring between said ring and said cap, whereby said cap and cup are retained against radial displacement and cushioned against axial compression in said housing,

a member extending axially from said tapered end of said plunger through said opening in said cap and having a radially extending surface facing away from said plunger and a pin projecting axially from the center of said surface,

a radially extending circular bimetallic disc having a central aperture receiving said pin, the high expansion side of said bimetallic disc abutting said radially extending surface and the low expansion side of said disc facing away from said surface whereby the position of said plunger is adjusted to compensate for changes in plunger position, coil resistance and permeability due to changes in temperature,

a circular plate member having its outer rim engaging the periphery of said bimetallic disc,

and pressure responsive means connected to said plate member and axially positioning said plunger to control said air gap and thereby control the re- 8 luctance of said flux path and thus control the inductance of said coil. 6. A transducer for converting a variable pressure signal to a variable inductive signal comprising a nylon spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening adjacent the large diameter end thereof,

an inductive coil wound about said spool,

a nickel plated steel sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annular recess receiving said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool,

a ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a centralopening receiving said other end of said sleeve, a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening,

a Teflon coated steel plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap,

said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap,

a cupped housing having an end wall and a peripheral wall, said end wall of said ferrite cup being disposed adjacent said end wall of said housing. and said peripheral wall of said housing surrounding said cylindrical wall of said ferrite cup, said end wall having an axial recess receiving and aligning said sleeve,

a retainer ring disposed adjacent said ferrite cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall,

a silicone rubber wafer disposed between said end wall of said ferrite cup and said end wall of said housing, and a silicone rubber wafer secured to said retainer ring between said ring and said cap, whereby said ferrite cap and cup are retained against radial displacement and cushioned against axial compression in said housing,

a brass member extending axially from said tapered end of said plunger through said opening in said cap and having a radially extending surface facing away from said plunger and a pin projecting axially from the center of said surface,

a radially extending circular bimetallic disc having a central aperture receiving said pin, the high expansion side of said bimetallic disc abutting said radially extending surface and the low expansion side of said disc facing away from said surface whereby the position of said plunger is adjusted to compensate for changes in plunger position, coil resistance and ferrite permeability due to changes in temperature,

a circular plate member having its outer rim engaging the periphery of said bimetallic disc,

an evacuated bellows having its exterior exposed to said variable pressure signal and having one end connected to said plate member for axially positioning saidplunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil,

spring means biasing said plunger toward a maximum inductance position,

an adjustable stop screw extending from said housing end wall into the other end of said sleeve and abutting said plunger to limit travel thereof against the bias of said spring 'means in a minimum inductance direction,

and a calibration screw abutting the opposite end of said bellows and adjustable to establish an initial position of said plunger. i

7. A transducer for converting a variable pressure signal to a variable inductive signal comprising:

an inductive coil,

a plunger disposed within said coil,

means associated with said plunger for defining a flux path through and about said coil,

an evacuated bellows having its exterior exposed to said variable pressure signal and having one end connected to said plunger for axially moving said plunger within said coil to thereby vary the inductance of said coil,

spring means biasing said plunger toward a maximum inductance position,

an adjustable stop screw abutting said plunger to limit travel thereof against the bias of said spring means in a minimum inductance direction,

a calibration screw abutting'the opposite end of said bellows,

flexible diaphragm means carrying said calibration screw,

a helical power spring having one end abutting said diaphragm to bias said diaphragm toward a maximum inductance position;

an adjustable stop screw abutting said diaphragm means to limit travel thereof in a maximum inductance direction, said diaphragm means being subjected on one side to substantially atmospheric pressure and on the other side to said variable pressure signal whereby said diaphragm means moves toward a minimum inductance position against the bias of said power spring in response to a decrease in said variable pressure signal below a predetermined value,

stop means abutting said diaphragm means to limit movement of said diaphragm in a minimum inductance direction,

and a screw abutting the other end of said power spring and adjustable for regulating the compression of said power spring to thereby establish said predetermined variable pressure signal value below which said diaphragm means moves toward a minimum inductance position,

said calibration screw being adjustable with respect to said diaphragm to establish an initial position of said plunger. 

1. A transducer for converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening adjacent the large diameter end thereof, a coil wound about said spool, a steel sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annular recess receiving said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool, a ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said other end of said sleeve, a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, a steel plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap, said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap, and pressure responsive means connected to said plunger for axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil.
 2. A transducer for converting a variable pressure signal to a variable inductive signal comprising: an inductive coil, a plunger disposed within said coil, means associated with said plunger for defining a flux path through and about said coil, pressure responsive means for axially moving said plunger within said coil to thereby vary the inductance of said coil, and bimetallic temperature responsive means connecting said pressure responsive means to said plunger for varying the axial position of said plunger with respect to said pressure responsive means to thereby compensate for variations in plunger position, coil resistance, and flux path permeability due to temperature variations.
 3. A transducer for converting a variable pressure signal to a variable inductive signal comprising: an inductive coil, a plunger disposed within said coil, means associated with said plunger for defining a flux path through and about said coil, said plunger being axially movable within said coil to thereby vary the inductance of said coil, a member connected to said plunger and having a radially extending surface facing away from said plunger, a radially extending circular bimetallic disc the opposite sides of which have differing rates of thermal expansion, one of said sides abutting said radially extending surface and the other of said sides facing away from said surface whereby the position of said plunger is adjusted to compensate for changes in temperature, a circular plate member having its outer rim engaging the periphery of said bimetallic disc, and pressure responsive means connected to said plate member for axially positioning said plunger within said coil to control the inductance of said coil.
 4. A transducer for converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough and an inductive coil wound thereabout, a sleeve received in said opening, one end of said sleeve extending axially beyond the end of said spool, a ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said end of said sleeve, a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, a plunger Guided in said sleeve and having a tapered end disposed adjacent said opening in said cap, said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap, a cupped housing having an end wall and a peripheral wall, said end wall of said ferrite cup being disposed adjacent said end wall of said housing and said peripheral wall of said housing surrounding said cylindrical wall of said ferrite cup, said end wall having an axial recess receiving and aligning said sleeve, a retainer ring disposed adjacent said ferrite cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall, a resilient wafer disposed between said end wall of said ferrite cup and said end wall of said housing, and a resilient wafer secured to said retainer ring between said ring and said cap, whereby said ferrite cap and cup are retained against radial displacement and cushioned against axial compression in said housing, and pressure responsive means connected to said plunger for axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil.
 5. A transducer for converting a variable pressure signal to a variable inductive signal comprising a spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening adjacent the large diameter end thereof, an inductive coil would about said spool, a sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annular recess receiving said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool, a cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said other end of said sleeve, a circular cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, a plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap, said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap, a cupped housing having an end wall and a peripheral wall, said end wall of said cup being disposed adjacent said end wall of said housing and said peripheral wall of said housing surrounding said cylindrical wall of said cup, said end wall having an axial recess receiving and aligning said sleeve, a retainer ring disposed adjacent said cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall, a resilient wafer disposed between said end wall of said cup and said end wall of said housing and a resilient wafer secured to said retainer ring between said ring and said cap, whereby said cap and cup are retained against radial displacement and cushioned against axial compression in said housing, a member extending axially from said tapered end of said plunger through said opening in said cap and having a radially extending surface facing away from said plunger and a pin projecting axially from the center of said surface, a radially extending circular bimetallic disc having a central aperture receiving said pin, the high expansion side of said bimetallic disc abutting said radially extending surface and the low expansion side of said disc facing away From said surface whereby the position of said plunger is adjusted to compensate for changes in plunger position, coil resistance and permeability due to changes in temperature, a circular plate member having its outer rim engaging the periphery of said bimetallic disc, and pressure responsive means connected to said plate member and axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil.
 6. A transducer for converting a variable pressure signal to a variable inductive signal comprising a nylon spool having an opening extending axially therethrough, said opening having a diameter at one end slightly larger than the diameter at the other end and having an annular shoulder providing a transition between said diameters, said spool including a projection extending radially into said opening adjacent the large diameter end thereof, an inductive coil wound about said spool, a nickel plated steel sleeve received in said opening and having one end abutting said shoulder, said sleeve having an annular recess receiving said projection whereby said sleeve is retained in said spool, the other end of said sleeve extending axially beyond the end of said spool, a ferrite cup having a circular end wall disposed at said end of said spool and a cylindrical wall of circular cross section surrounding said spool, said end wall having a central opening receiving said other end of said sleeve, a circular ferrite cap disposed at the other end of said spool and secured to said cup, said cap having a central opening, a Teflon coated steel plunger guided in said sleeve and having a tapered end disposed adjacent said opening in said cap, said plunger, said cap, said cup, and said sleeve defining a flux path through and about said coil, said tapered end of said plunger defining an air gap with said cap, a cupped housing having an end wall and a peripheral wall, said end wall of said ferrite cup being disposed adjacent said end wall of said housing and said peripheral wall of said housing surrounding said cylindrical wall of said ferrite cup, said end wall having an axial recess receiving and aligning said sleeve, a retainer ring disposed adjacent said ferrite cap and having a plurality of radially extending fingers engaging said peripheral wall to prevent axial movement of said ferrite cap and cup away from said housing end wall, a silicone rubber wafer disposed between said end wall of said ferrite cup and said end wall of said housing, and a silicone rubber wafer secured to said retainer ring between said ring and said cap, whereby said ferrite cap and cup are retained against radial displacement and cushioned against axial compression in said housing, a brass member extending axially from said tapered end of said plunger through said opening in said cap and having a radially extending surface facing away from said plunger and a pin projecting axially from the center of said surface, a radially extending circular bimetallic disc having a central aperture receiving said pin, the high expansion side of said bimetallic disc abutting said radially extending surface and the low expansion side of said disc facing away from said surface whereby the position of said plunger is adjusted to compensate for changes in plunger position, coil resistance and ferrite permeability due to changes in temperature, a circular plate member having its outer rim engaging the periphery of said bimetallic disc, an evacuated bellows having its exterior exposed to said variable pressure signal and having one end connected to said plate member for axially positioning said plunger to control said air gap and thereby control the reluctance of said flux path and thus control the inductance of said coil, spring means biasing said plunger toward a maximum inductance position, an adjustable stop screw extending from said housing end wall into the other end of said sleeve and abutting said plunger to limit travel thereof against the bias of said spring means in a minimum inductance direction, and a calibration screw abutting the opposite end of said bellows and adjustable to establish an initial position of said plunger.
 7. A transducer for converting a variable pressure signal to a variable inductive signal comprising: an inductive coil, a plunger disposed within said coil, means associated with said plunger for defining a flux path through and about said coil, an evacuated bellows having its exterior exposed to said variable pressure signal and having one end connected to said plunger for axially moving said plunger within said coil to thereby vary the inductance of said coil, spring means biasing said plunger toward a maximum inductance position, an adjustable stop screw abutting said plunger to limit travel thereof against the bias of said spring means in a minimum inductance direction, a calibration screw abutting the opposite end of said bellows, flexible diaphragm means carrying said calibration screw, a helical power spring having one end abutting said diaphragm to bias said diaphragm toward a maximum inductance position, an adjustable stop screw abutting said diaphragm means to limit travel thereof in a maximum inductance direction, said diaphragm means being subjected on one side to substantially atmospheric pressure and on the other side to said variable pressure signal whereby said diaphragm means moves toward a minimum inductance position against the bias of said power spring in response to a decrease in said variable pressure signal below a predetermined value, stop means abutting said diaphragm means to limit movement of said diaphragm in a minimum inductance direction, and a screw abutting the other end of said power spring and adjustable for regulating the compression of said power spring to thereby establish said predetermined variable pressure signal value below which said diaphragm means moves toward a minimum inductance position, said calibration screw being adjustable with respect to said diaphragm to establish an initial position of said plunger. 